A filter screen assembly conventionally comprises a steel frame with one or more filtering meshes secured and tensioned over its area. In use, the mesh can become damaged, creating holes through which can pass material which the mesh is intended to hold back. Replacement of the mesh in such an event is expensive and time consuming, requiring removal of the frame from the machine, removal of the damaged mesh, replacement by a new mesh and tensioning of the latter, and replacement of the frame in the machine.
It is known to effect patching of a damaged mesh with metal sheet patches or, in the case of a multiple mesh assembly, to bond the two meshes together at the site of damage. The former procedure results in diminished throughput, whilst the latter results in reduced efficiency of the so-called blinding action which is intended to take place in use, whereby the differential vibrations of the two meshes serve to dislodge material tending to clog the upper mesh. Both procedures, whilst more economical than replacement of a complete mesh, are just as time consuming as a complete mesh replacement.
It has been proposed in U.S. Pat. No. 4219412 to Hassell to provide a screen assembly comprising a latticed support frame into the apertures of which are force fitted sieve elements. Such elements are made of a synthetic plastics material, such as polyurethane, and no tensioning of the sieve occurs. In fact, the disclosed fitting of the element in each aperture is such that if there were any tensioning in the sieve, it would tend to be reduced after fitting. Furthermore, there is no teaching in Hassell of any of the sieve elements being replaceable, eg for repair. Indeed the Hassell system seems to be aimed at the manufacture of the original product--not the repair or replacement of brother parts.
In U.S. Pat. No. 7875065 to Rosenblum there is disclosed a tensioned circular filter screen adapted to be force fitted over an annular lip of a frame to increase the tension in the screen. The screen is intended to be permanently secured, and so cannot be readily removed from the frame. There is no suggestion in Rosenblum that a plurality of such screens could be used as sieve elements in a latticed support frame as in Hassell, even if the force fitting were compatible.
It is therefore an object of the present invention to provide an improved filter screen assembly in which the filter mesh elements are readily replaceable in the principal frame, and are adapted in fitting to at least maintain the tension in the mesh.